Currently, the presence of biologically active agents such as bacteria in a patient's body fluid, and especially in blood, is determined using blood culture vials. A small quantity of blood is injected through an enclosing rubber septum into a sterile vial containing a culture medium and the vial is then incubated and monitored for bacterial growth.
Common visual inspection of the culture vial then involves monitoring the turbidity or observing eventual color changes of the liquid suspension within the vial. Known instrument methods can also be used to detect changes in the carbon dioxide content of the culture bottles, which is a metabolic byproduct of the bacterial growth. Monitoring the carbon dioxide content can be accomplished by methods well established in the art; however, most of these methods require invasive procedures which can result in the well-known problem of cross-contamination within the vial.
One solution to the above problems includes the use of a non-invasive infrared microorganism detection instrument in which special vials having infrared-transmitting windows are utilized. These vials, however, are relatively expensive. In yet another solution, glass vials are transferred to an infrared spectrometer by an automated manipulator arm and measured through the glass vial. The disadvantage of this system is that, due to the high infrared absorption of glass, small changes in the glass wall thickness can generate large errors in the measured headspace gas absorption. These problems can be partly reduced by utilizing high-quality glass vials, but this measure results in relatively high vial cost.
Still other solutions have included the use of chemical sensors disposed inside the vial. These sensors respond to changes in the carbon dioxide concentration in the liquid phase by changing their color or by changing their fluorescence intensity. These techniques are based on light intensity measurements and require spectral filtering in the excitation and/or emission signals. However, in such solutions, errors can occur if any of the light source, photodetector, filters, or sensor show aging effects over time, which would vary the intensity response.
Accordingly, a need exists for a system and method to measure a change in a sample in a non-invasive manner to avoid contamination of the sample.